A slide projector, through which an image of a transparency is projected on a screen, generally comprises a projection lens which can extend through an aperture in the casing or housing of the slide projector and is seated in a focusing system disposed internally of this housing. This applies also to the Ektagraphic III Kodak projector as it does to many modern slide projectors.
It is frequently desirable to substitute for the lenses applied with the projector other lenses of a variety of apertures or focal length, and even zoom lenses, while ensuring that even heavy lenses can be accommodated with stability and can be focused.